A battery converts stored chemical energy to electrical energy, which may be conveyed as a voltage source. As a battery ages its storage capacity will decrease (i.e., fade) between a Beginning Of Life (BOL) and an End Of Life (EOL). Therefore, observations of battery parameters such as cycle rate, cumulative cycling time, and storage capacity may be helpful in determining an overall State Of Health (SOH) of a battery over its service life.
Battery fuel gauges, battery capacity monitors, and battery status monitors attempt to report battery capacities and give the user an idea of remaining capacity. Conventionally, battery capacity is estimated by current integration, voltage monitoring, or combinations thereof.
Current integration, or coulomb counting as it is commonly called, monitors the battery's available stored charge by measuring the amount of charge that enters and exits the battery through normal cycling. The basis for this approach is that if all charge and discharge currents are known, the amount of coulometric capacity will be known.
Voltage monitoring methods are based on the recognized relationship between the battery terminal voltage and the remaining capacity. All that is required is voltage measurement of the battery terminals to acquire a rough idea of the State Of Charge (SOC) of the battery.
Voltage monitoring may show errors when measurements are taken with load on the battery. When a load is applied, the voltage drop due to the internal impedance of the battery distorts battery voltage. For many batteries, such as lithium-ion batteries, even after the load is removed, slow time constants and relaxation processes may continue to change the battery voltage for hours. Also, some battery chemistries (e.g., nickel metal-hydride) exhibit a strong voltaic hysteresis, which hinders the possibility of using voltage to track capacity.
SOC algorithms and measurement techniques are well known, but methods to predict battery life are less common. There is a need for systems and methods that provide a modeling capability that more accurately determines, tracks, diagnoses, and predicts capacity loss in electrochemical cells and batteries formed therefrom.